Bone anchor assemblies can be used in orthopedic surgery to fix bone during healing, fusion, deformity correction, or other processes. In spinal surgery, for example, bone anchor assemblies can be used to secure a spinal fixation element to one or more vertebrae to rigidly or dynamically stabilize the spine.
Bone anchor assemblies traditionally include an elongate shank with a proximal head and a receiver member in which the proximal head of the shank is received. The receiver member can also receive a spinal stabilization element therein, such as a spinal rod. The receiver member can be configured to allow for various types of motion of the shank with respect thereto (e.g., polyaxial motion, uniplanar motion, favored angle motion, etc.).
The capacity for the shank to angulate with respect to the receiver member can make it difficult to visually assess the orientation or trajectory of the shank, for example when the shank is at least partially advanced into bone or during minimally-invasive procedures in which visibility of the shank is limited. In addition, while motion between the receiver member and the shank can be desirable during some portions of a surgical procedure, it can be undesirable during other portions (e.g., during derotation maneuvers or during insertion of the bone anchor assembly).